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1.2 +++ b/src/share/vm/gc_implementation/g1/g1StringDedup.hpp Tue Mar 18 19:07:22 2014 +0100
1.3 @@ -0,0 +1,202 @@
1.4 +/*
1.5 + * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
1.6 + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.7 + *
1.8 + * This code is free software; you can redistribute it and/or modify it
1.9 + * under the terms of the GNU General Public License version 2 only, as
1.10 + * published by the Free Software Foundation.
1.11 + *
1.12 + * This code is distributed in the hope that it will be useful, but WITHOUT
1.13 + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
1.14 + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
1.15 + * version 2 for more details (a copy is included in the LICENSE file that
1.16 + * accompanied this code).
1.17 + *
1.18 + * You should have received a copy of the GNU General Public License version
1.19 + * 2 along with this work; if not, write to the Free Software Foundation,
1.20 + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
1.21 + *
1.22 + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
1.23 + * or visit www.oracle.com if you need additional information or have any
1.24 + * questions.
1.25 + *
1.26 + */
1.27 +
1.28 +#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
1.29 +#define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
1.30 +
1.31 +//
1.32 +// String Deduplication
1.33 +//
1.34 +// String deduplication aims to reduce the heap live-set by deduplicating identical
1.35 +// instances of String so that they share the same backing character array.
1.36 +//
1.37 +// The deduplication process is divided in two main parts, 1) finding the objects to
1.38 +// deduplicate, and 2) deduplicating those objects. The first part is done as part of
1.39 +// a normal GC cycle when objects are marked or evacuated. At this time a check is
1.40 +// applied on each object to check if it is a candidate for deduplication. If so, the
1.41 +// object is placed on the deduplication queue for later processing. The second part,
1.42 +// processing the objects on the deduplication queue, is a concurrent phase which
1.43 +// starts right after the stop-the-wold marking/evacuation phase. This phase is
1.44 +// executed by the deduplication thread, which pulls deduplication candidates of the
1.45 +// deduplication queue and tries to deduplicate them.
1.46 +//
1.47 +// A deduplication hashtable is used to keep track of all unique character arrays
1.48 +// used by String objects. When deduplicating, a lookup is made in this table to see
1.49 +// if there is already an identical character array somewhere on the heap. If so, the
1.50 +// String object is adjusted to point to that character array, releasing the reference
1.51 +// to the original array allowing it to eventually be garbage collected. If the lookup
1.52 +// fails the character array is instead inserted into the hashtable so that this array
1.53 +// can be shared at some point in the future.
1.54 +//
1.55 +// Candidate selection
1.56 +//
1.57 +// An object is considered a deduplication candidate if all of the following
1.58 +// statements are true:
1.59 +//
1.60 +// - The object is an instance of java.lang.String
1.61 +//
1.62 +// - The object is being evacuated from a young heap region
1.63 +//
1.64 +// - The object is being evacuated to a young/survivor heap region and the
1.65 +// object's age is equal to the deduplication age threshold
1.66 +//
1.67 +// or
1.68 +//
1.69 +// The object is being evacuated to an old heap region and the object's age is
1.70 +// less than the deduplication age threshold
1.71 +//
1.72 +// Once an string object has been promoted to an old region, or its age is higher
1.73 +// than the deduplication age threshold, is will never become a candidate again.
1.74 +// This approach avoids making the same object a candidate more than once.
1.75 +//
1.76 +// Interned strings are a bit special. They are explicitly deduplicated just before
1.77 +// being inserted into the StringTable (to avoid counteracting C2 optimizations done
1.78 +// on string literals), then they also become deduplication candidates if they reach
1.79 +// the deduplication age threshold or are evacuated to an old heap region. The second
1.80 +// attempt to deduplicate such strings will be in vain, but we have no fast way of
1.81 +// filtering them out. This has not shown to be a problem, as the number of interned
1.82 +// strings is usually dwarfed by the number of normal (non-interned) strings.
1.83 +//
1.84 +// For additional information on string deduplication, please see JEP 192,
1.85 +// http://openjdk.java.net/jeps/192
1.86 +//
1.87 +
1.88 +#include "memory/allocation.hpp"
1.89 +#include "oops/oop.hpp"
1.90 +
1.91 +class OopClosure;
1.92 +class BoolObjectClosure;
1.93 +class ThreadClosure;
1.94 +class outputStream;
1.95 +class G1StringDedupTable;
1.96 +
1.97 +//
1.98 +// Main interface for interacting with string deduplication.
1.99 +//
1.100 +class G1StringDedup : public AllStatic {
1.101 +private:
1.102 + // Single state for checking if both G1 and string deduplication is enabled.
1.103 + static bool _enabled;
1.104 +
1.105 + // Candidate selection policies, returns true if the given object is
1.106 + // candidate for string deduplication.
1.107 + static bool is_candidate_from_mark(oop obj);
1.108 + static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
1.109 +
1.110 +public:
1.111 + // Returns true if both G1 and string deduplication is enabled.
1.112 + static bool is_enabled() {
1.113 + return _enabled;
1.114 + }
1.115 +
1.116 + static void initialize();
1.117 +
1.118 + // Immediately deduplicates the given String object, bypassing the
1.119 + // the deduplication queue.
1.120 + static void deduplicate(oop java_string);
1.121 +
1.122 + // Enqueues a deduplication candidate for later processing by the deduplication
1.123 + // thread. Before enqueuing, these functions apply the appropriate candidate
1.124 + // selection policy to filters out non-candidates.
1.125 + static void enqueue_from_mark(oop java_string);
1.126 + static void enqueue_from_evacuation(bool from_young, bool to_young,
1.127 + unsigned int queue, oop java_string);
1.128 +
1.129 + static void oops_do(OopClosure* keep_alive);
1.130 + static void unlink(BoolObjectClosure* is_alive);
1.131 + static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,
1.132 + bool allow_resize_and_rehash = true);
1.133 +
1.134 + static void threads_do(ThreadClosure* tc);
1.135 + static void print_worker_threads_on(outputStream* st);
1.136 + static void verify();
1.137 +};
1.138 +
1.139 +//
1.140 +// This closure encapsulates the state and the closures needed when scanning
1.141 +// the deduplication queue and table during the unlink_or_oops_do() operation.
1.142 +// A single instance of this closure is created and then shared by all worker
1.143 +// threads participating in the scan. The _next_queue and _next_bucket fields
1.144 +// provide a simple mechanism for GC workers to claim exclusive access to a
1.145 +// queue or a table partition.
1.146 +//
1.147 +class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {
1.148 +private:
1.149 + BoolObjectClosure* _is_alive;
1.150 + OopClosure* _keep_alive;
1.151 + G1StringDedupTable* _resized_table;
1.152 + G1StringDedupTable* _rehashed_table;
1.153 + size_t _next_queue;
1.154 + size_t _next_bucket;
1.155 +
1.156 +public:
1.157 + G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
1.158 + OopClosure* keep_alive,
1.159 + bool allow_resize_and_rehash);
1.160 + ~G1StringDedupUnlinkOrOopsDoClosure();
1.161 +
1.162 + bool is_resizing() {
1.163 + return _resized_table != NULL;
1.164 + }
1.165 +
1.166 + G1StringDedupTable* resized_table() {
1.167 + return _resized_table;
1.168 + }
1.169 +
1.170 + bool is_rehashing() {
1.171 + return _rehashed_table != NULL;
1.172 + }
1.173 +
1.174 + // Atomically claims the next available queue for exclusive access by
1.175 + // the current thread. Returns the queue number of the claimed queue.
1.176 + size_t claim_queue() {
1.177 + return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;
1.178 + }
1.179 +
1.180 + // Atomically claims the next available table partition for exclusive
1.181 + // access by the current thread. Returns the table bucket number where
1.182 + // the claimed partition starts.
1.183 + size_t claim_table_partition(size_t partition_size) {
1.184 + return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;
1.185 + }
1.186 +
1.187 + // Applies and returns the result from the is_alive closure, or
1.188 + // returns true if no such closure was provided.
1.189 + bool is_alive(oop o) {
1.190 + if (_is_alive != NULL) {
1.191 + return _is_alive->do_object_b(o);
1.192 + }
1.193 + return true;
1.194 + }
1.195 +
1.196 + // Applies the keep_alive closure, or does nothing if no such
1.197 + // closure was provided.
1.198 + void keep_alive(oop* p) {
1.199 + if (_keep_alive != NULL) {
1.200 + _keep_alive->do_oop(p);
1.201 + }
1.202 + }
1.203 +};
1.204 +
1.205 +#endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
